The West is striving to limit the fallout from a deadly Nato air raid on Pakistani border troops, but reports the soldiers opened fire first on US and Afghan forces risked stoking new tensions.

The West is striving to limit the fallout from a deadly Nato air raid on Pakistani border troops, but reports the soldiers opened fire first on US and Afghan forces risked stoking new tensions.

Pakistan is simmering over the killings of the 24 soldiers, with fiery weekend protests denouncing the assault by Nato helicopters and fighter jets on two military posts on the Afghan border early on Saturday.

The United States, which depends on Pakistan as a vital lifeline to supply 130,000 foreign troops fighting in landlocked Afghanistan, on Sunday scrambled to salvage the alliance, backing a full inquiry and expressing condolences.

Nato secretary general Anders Fogh Rasmussen also sought to soothe Islamabad's rage, but stopped short of issuing a full apology for the "tragic, unintended" killings.

In retaliation for the raid, Islamabad has blocked Nato convoys from crossing into Afghanistan, ordered a review of its alliance with the US and mulled whether to boycott a key conference on Afghanistan next month.

Hundreds of enraged Pakistanis took to the streets Sunday, burning an effigy of President Barack Obama and setting fire to US flags across the country of 167 million where opposition to the government's US alliance is rampant.

Pakistan says the attack was unprovoked.

Foreign minister Hina Rabbani Khar telephoned US secretary of state Hillary Clinton on Sunday to convey a "deep sense of rage" as a joint funeral was held for the dead soldiers, their coffins draped in the national flag.

But a report in Monday's Wall Street Journal -- denied by Islamabad -- said the Nato jets and helicopters responded to firing from a Pakistani post on the ill-defined Afghan border.

The article, which followed a similar report by Britain's Guardian newspaper, cited three Afghan officials and one Western official as saying the air raid was called in to shield allied forces targeting Taliban fighters.

Nato and Afghan forces "were fired on from a Pakistani army base", the unnamed Western official told the Wall Street Journal. "It was a defensive action."

An Afghan official in Kabul was quoted as saying: "There was firing coming from the position against Afghan army soldiers who requested support and this is what happened."

The official added that the government in Kabul believes the fire came from the Pakistani military base -- and not from insurgents in the area.

There was no official US response to the report.

The latest tensions have erupted months after the fraught US-Pakistan alliance was plunged into crisis by the killing in May of al Qaeda leader Osama bin Laden north of Islamabad by US special forces.

On the Fox News on Sunday talk show, US lawmakers vented frustration over Pakistan, with Republican senator Jon Kyl demanding Islamabad cooperate with the United States in order to maintain billions of dollars in financial aid.

Senator Dick Durbin, a top Democrat, offered condolences but said US soldiers were caught in a "diplomatic morass between the incompetence and corruption in Afghanistan, and complicity in parts of Pakistan".

But John Bolton, a former US ambassador to the United Nations who was a hawkish foreign policy advisor to president George W. Bush, laid bare the dilemma for Washington in handling nuclear-armed Pakistan.

"While it is tempting for many people to say we ought to throw the Pakistanis over the side and stop giving them aid... as long as that country has nuclear weapons that could fall into the hands of radicals and be a threat worldwide, they have incredible leverage," he said.

The United States in 2009 approved a huge five-year, $7.5 billion civilian assistance package for Pakistan, but some US lawmakers want to cut civilian aid due to concerns over extremism.